This invention relates to structural composite laminates and the problem of preventing delamination at load-bearing apertures in such structural components.
The structural integrity of composite laminates is severely compromised when such laminates are drilled or cut such as for purposes of attachment. An aperture in such a laminate tends to especially compromise the axial or Z-axis integrity of the laminate and provides a site for delamination of the composite laminate.
In high-performance applications such as aerospace structures a typical composite may consist of a mat of interwoven high modulus filaments impregnated with a polymer. The drilling of such a laminate to provide an aperture destroys the continuity of the structural filaments contained within the composite. Conventional metal fasteners inserted into such apertures are unsuitable for attachment purposes. The difference in thermal coefficient of expansion of the materials of the composite and the fasteners as well as the differences in hardness will eventually destroy the laminate. The instant invention provides a composite reinforcing grommet of materials similar to that of the composite laminate, thus avoiding thermal expansion problems. Such a grommet is further capable of being co-bonded to the composite material. The polymer impregnated braided filaments of such grommets are capable of being radially expanded to form flanges to secure composite laminates therebetween and thus provide axial or Z-axis integrity. The grommet then provides a suitable attachment mechanism for conventional fasteners.